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LETTERS TO THE EDITOR
Department of Biomedical and Pharmaceutical Sciences University of Rhode Island, Kingston, Rhode Island (D.S., J.Y., D.Y., L.Y., F.A., B.Y.); and CellzDirect, Austin, Texas (E.L.L., C.B.)
Received January 17, 2007 ; accepted January 22, 2007.
) state that the clopidogrel-oseltamivir interaction (in vivo) is remote because the concentrations used in the in vitro metabolism study were much higher than the maximal plasma concentrations (Cmax) for both oseltamivir and clopidogrel. To support the argument, they refer to the unbound Cmax values observed in human pharmacokinetic studies. This argument is invalid because both agents undergo extensive hydrolysis in the liver during the absorption phase, and the observed plasma concentrations of the parent drugs represent only a small fraction of the absorbed dose (escaped from the first pass metabolism) (He et al., 1999). Based on the area under the concentration (AUC) time curve, the exposure to oseltamivir carboxylate was 20-fold higher than the exposure to the prodrug oseltamivir (He et al., 1999; Oo et al., 2002). Likewise, Taubert et al. (2004) reported that the AUC of clopidogrel carboxylate was 1500-fold greater than the AUC of clopidogrel. Such ratios with the predominance of the metabolites suggest that, in the absence of first pass effect, the initial plasma concentrations of the parent drugs would have been much higher than the observed plasma concentrations.
Although it is difficult to predict the exact concentrations in the liver during the first passage, estimates can be made with comfortable confidence according to the volume and flow rate of the portal blood. In healthy adults, the volume of portal blood space is approximately 450 ml, and portal blood flow rate is 
600 ml/min (Hofmann et al., 1983). Therefore, in the first 30 min after oral ingestion of a 75-mg dose of oseltamivir, approximately 18 liters of portal vein blood would pass through the liver. Assuming complete absorption, the concentration reaching the liver can be approximately 4100 µg/l or 13 µM, which is close to the concentrations used in the study reported by Shi et al. (2006). Intuitively, such a small time window may have only limited pharmacokinetic impact. Based on the fact that the AUC value of oseltamivir carboxylate is predominantly greater than that of oseltamivir and that the metabolite rapidly exceeds the parent drug in the blood (He et al., 1999; Abe et al., 2006), the majority of absorbed oseltamivir is hydrolyzed during the first pass of the liver. Apparently, the effective hydrolysis is attributed by the initial high concentrations in the liver. However, when clopidogrel is coadministered, the initial hydrolysis is inhibited. As a result, the activation of oseltamivir would rely on the subsequent exposure via the systemic circulation. The hydrolysis upon the systemic circulation is less effective, because the plasma concentration is much lower than the initial hepatic concentration. Therefore, only the initial inhibition alone may have profound effect on the overall activation of oseltamivir. The inhibited hydrolysis, on the other hand, would increase the blood concentration of oseltamivir, thus leading to increased elimination in the form of parent drug.
There is a growing interest in developing a strategy to predict clinical drug-drug interactions based on in vitro metabolism data. The widely studied system is the metabolism catalyzed by the cytochrome P450 superfamily proteins. In this system, the ratio of the concentration of an inhibitor (I) over its inhibitory constant (Ki) provides useful predictive information (Wienkers and Heath, 2005; Bachmann, 2006). However, in some cases, the predictability is confounded by certain variables (e.g., the selection of inhibitor concentrations). It remains to be determined whether this method provides similar predictability for carboxylesterase-based metabolism. Nevertheless, Fowler et al. (2007) state that they used this procedure and predicted that the clinical interaction between oseltamivir and clopidogrel is remote. Apparently, they used the plasma but not the liver concentrations for their predication. Strictly speaking, metabolism-based interaction is determined by the relative concentrations at the site of the enzyme involved in the metabolism. In this case, the liver concentrations should be used.
It should be noted that in vitro metabolism studies are usually conducted with relatively high concentrations in order for the assay conditions to be properly controlled. The United States Food and Drug Administration (2006) recommends that inhibition studies be performed with a substrate concentration below its Km and no more than 10 to 30% depletion of substrate (i.e., oseltamivir) and inhibitor (i.e., clopidogrel) during the assay. In addition, the inhibition of oseltamivir hydrolysis by clopidogrel is competitive in nature (Shi et al., 2006); thus, the inhibition should be dictated by the concentration ratio (clopidogrel over oseltamivir) but not by the absolute concentrations. It is reasonable to believe that similar magnitude of inhibition would have been detected with substantially lower concentrations of oseltamivir and clopidogrel (probably with less enzyme proteins).
The cell-based assay provides important mechanistic insight regarding oseltamivir-induced toxicity. Although as high as 320 µM was used for the toxicity study, significant decreases on cell viability were detected at 10 µM (Shi et al., 2006). More importantly, the toxicity was detected in oseltamivir-hydrolytic cells only, suggesting that the intracellular accumulation of the hydrolytic metabolite is responsible for the toxicity. Fowler et al. (2007) state that studies in rats and monkeys detected no toxicity, even though these animals produced high plasma concentrations of the hydrolytic metabolite (229-327 µM). The precise mechanism on the discrepancy remains to be determined. It is likely that the whole animal studies used toxicological endpoints less sensitive than the cell-based assay. Alternatively, these animals produced the metabolite by serum carboxylesterases, and the negatively charged metabolite stayed in the plasma. Li et al. (2005) reported that there is no serum carboxylesterase in humans. In contrast, animals such as rats express high levels of serum carboxylesterases (Yan et al., 1995), and the serum carboxylesterases presumably hydrolyze oseltamivir as well.
While producing a large amount of hydrolytic metabolite of oseltamivir, hepatocytes may exhibit less toxicity because these cells usually express high levels of transporters. Based on the predominance of the metabolite over the parent drug in the blood (He et al., 1999; Oo et al., 2002), the hepatic effluxing of the metabolite is very effective. On the other hand, decreased hepatic hydrolysis may increase the concentrations of oseltamivir in other types of cells and induce toxicity, particularly in the cells that efficiently hydrolyze oseltamivir but lack the ability of effluxing the metabolite. In humans, there are several mechanisms that may support reduced hepatic hydrolysis of oseltamivir. First, certain drugs concurrently administered may profoundly inhibit the hydrolysis of oseltamivir as suggested with clopidogrel (Shi et al., 2006). Second, people express polymorphic variants of HCE1 with markedly decreased ability to hydrolyze this agent (Shi et al., 2006). And finally, certain cytokines released during influenza infection may down-regulate the expression of HCE1.
In summary, the article by Shi et al. (2006) reports several important findings regarding the metabolism of oseltamivir, cytotoxicity, and potential interaction with clopidogrel. As discussed in the article, although the in vitro metabolism study revealed profound inhibition, in vivo interaction between oseltamivir and clopidogrel remains to be established in humans. The cell-based assay provides a molecular explanation on how oseltamivir may induce cytotoxicity, but such explanation may apply to specific types of cells that hydrolyze oseltamivir but lack the ability of effluxing the metabolite.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
ABBREVIATIONS: AUC, area under the concentration; HCE1, human carboxylesterase.
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References |
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